# Opamp current noise

Started by July 20, 2012
```Opamp current noise.

I got a call the other day from someone seeing =91too much=92 warm up
drift in the Johnson noise voltage from our Noise fundamentals
instrument.
http://www.teachspin.com/instruments/noise/index.shtml

I had them send it back to me.  (A few hundred dollar learning
experience)
After much head banging, it turns out that the voltage noise of the
first stage opamp increases by ~3-5% in an exponential way with a ~10
=96 20 minute time constant.  (I haven=92t had time for a carful
measurement.)

Anyway, along the way I became worried about the current noise of the
opamp (opa134).
I=92m wondering if any one has an idea on how to measure this.
I think Phil H. mumbled something about a correlation technique when I
mentioned this a year or so ago.  But now I can=92t see how to make that
work.

My current idea (NPI) is to just switch in a second similar opamp.
Like this,

switch
|
V   |\
+-/.---|+\
|      |  >-+-->(output to nowhere)
|    +-|-/  |
|    | |/   |
|    +------+   |\
+---------------|+\
|               |  >-+-->(output to more gain,
R             +-|-/  |    filter, multiplier)
R             | |/   |
R             +------+
|
GND

And measure the increase in noise for different R=92s.
Will this work?
Any other ideas?

Thanks,

George H.
```
```On Fri, 20 Jul 2012 08:01:50 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>Opamp current noise.
>
>I got a call the other day from someone seeing &#2013266065;too much&#2013266066; warm up
>drift in the Johnson noise voltage from our Noise fundamentals
>instrument.
>http://www.teachspin.com/instruments/noise/index.shtml
>
>I had them send it back to me.  (A few hundred dollar learning
>experience)
>After much head banging, it turns out that the voltage noise of the
>first stage opamp increases by ~3-5% in an exponential way with a ~10
>&#2013266070; 20 minute time constant.  (I haven&#2013266066;t had time for a carful
>measurement.)
>
>Anyway, along the way I became worried about the current noise of the
>opamp (opa134).
>I&#2013266066;m wondering if any one has an idea on how to measure this.
>I think Phil H. mumbled something about a correlation technique when I
>mentioned this a year or so ago.  But now I can&#2013266066;t see how to make that
>work.
>
>My current idea (NPI) is to just switch in a second similar opamp.
>Like this,
>
>   switch
>     |
>     V   |\
>  +-/.---|+\
>  |      |  >-+-->(output to nowhere)
>  |    +-|-/  |
>  |    | |/   |
>  |    +------+   |\
>  +---------------|+\
>  |               |  >-+-->(output to more gain,
>  R             +-|-/  |    filter, multiplier)
>  R             | |/   |
>  R             +------+
>  |
> GND
>
>And measure the increase in noise for different R&#2013266066;s.
>Will this work?
>Any other ideas?
>
>Thanks,
>
>George H.

This AN has some useful stuff:-
http://www.intersil.com/content/dam/Intersil/documents/an15/an1560.pdf

```
```On Jul 20, 8:01=A0am, George Herold <gher...@teachspin.com> wrote:
> Opamp current noise.
>
> I got a call the other day from someone seeing =91too much=92 warm up
> drift in the Johnson noise voltage from our Noise fundamentals
> instrument.http://www.teachspin.com/instruments/noise/index.shtml
>
> I had them send it back to me. =A0(A few hundred dollar learning
> experience)
> After much head banging, it turns out that the voltage noise of the
> first stage opamp increases by ~3-5% in an exponential way with a ~10
> =96 20 minute time constant. =A0(I haven=92t had time for a carful
> measurement.)
>
> Anyway, along the way I became worried about the current noise of the
> opamp (opa134).
> I=92m wondering if any one has an idea on how to measure this.
> I think Phil H. mumbled something about a correlation technique when I
> mentioned this a year or so ago. =A0But now I can=92t see how to make tha=
t
> work.
>
> My current idea (NPI) is to just switch in a second similar opamp.
> Like this,
>
> =A0 =A0switch
> =A0 =A0 =A0|
> =A0 =A0 =A0V =A0 |\
> =A0 +-/.---|+\
> =A0 | =A0 =A0 =A0| =A0>-+-->(output to nowhere)
> =A0 | =A0 =A0+-|-/ =A0|
> =A0 | =A0 =A0| |/ =A0 |
> =A0 | =A0 =A0+------+ =A0 |\
> =A0 +---------------|+\
> =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0>-+-->(output to more gain,
> =A0 R =A0 =A0 =A0 =A0 =A0 =A0 +-|-/ =A0| =A0 =A0filter, multiplier)
> =A0 R =A0 =A0 =A0 =A0 =A0 =A0 | |/ =A0 |
> =A0 R =A0 =A0 =A0 =A0 =A0 =A0 +------+
> =A0 |
> =A0GND
>
> And measure the increase in noise for different R=92s.
> Will this work?
> Any other ideas?
>
> Thanks,
>
> George H.

I'm trying to remember what my 24 bit sound card's noise density is,
but 17nV/rtHz comes to mind.  If put a LT1115 preamp in there, you
could get down below 2nV/rtHz and use that as an instrument to measure
the noise density function.

Plus you get a free tracking generator output along with it.
```
```On Jul 20, 11:46=A0am, Spehro Pefhany
<speffS...@interlogDOTyou.knowwhat> wrote:
> On Fri, 20 Jul 2012 08:01:50 -0700 (PDT), George Herold
>
>
>
>
>
> <gher...@teachspin.com> wrote:
> >Opamp current noise.
>
> >I got a call the other day from someone seeing =91too much=92 warm up
> >drift in the Johnson noise voltage from our Noise fundamentals
> >instrument.
> >http://www.teachspin.com/instruments/noise/index.shtml
>
> >I had them send it back to me. =A0(A few hundred dollar learning
> >experience)
> >After much head banging, it turns out that the voltage noise of the
> >first stage opamp increases by ~3-5% in an exponential way with a ~10
> >=96 20 minute time constant. =A0(I haven=92t had time for a carful
> >measurement.)
>
> >Anyway, along the way I became worried about the current noise of the
> >opamp (opa134).
> >I=92m wondering if any one has an idea on how to measure this.
> >I think Phil H. mumbled something about a correlation technique when I
> >mentioned this a year or so ago. =A0But now I can=92t see how to make th=
at
> >work.
>
> >My current idea (NPI) is to just switch in a second similar opamp.
> >Like this,
>
> > =A0 switch
> > =A0 =A0 |
> > =A0 =A0 V =A0 |\
> > =A0+-/.---|+\
> > =A0| =A0 =A0 =A0| =A0>-+-->(output to nowhere)
> > =A0| =A0 =A0+-|-/ =A0|
> > =A0| =A0 =A0| |/ =A0 |
> > =A0| =A0 =A0+------+ =A0 |\
> > =A0+---------------|+\
> > =A0| =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0>-+-->(output to more gain,
> > =A0R =A0 =A0 =A0 =A0 =A0 =A0 +-|-/ =A0| =A0 =A0filter, multiplier)
> > =A0R =A0 =A0 =A0 =A0 =A0 =A0 | |/ =A0 |
> > =A0R =A0 =A0 =A0 =A0 =A0 =A0 +------+
> > =A0|
> > GND
>
> >And measure the increase in noise for different R=92s.
> >Will this work?
> >Any other ideas?
>
> >Thanks,
>
> >George H.
>
> This AN has some useful stuff:-http://www.intersil.com/content/dam/Inters=
il/documents/an15/an1560.pdf- Hide quoted text -
>
> - Show quoted text -

Thanks Spehro,
The current noise gets =91trouble some=92 out beyond 100kHz.
See figure bottom of page 4.

Making a big R kills the bandwidth.  I=92ve got switches and terminal
blocks in the front end, which give about 7pF of input C.  Making some
sort of difference measurement would be ideal.
(It's fairly easy to see 1% change in the noise.)

George H.

```
```On Jul 20, 12:30=A0pm, Robert Macy <robert.a.m...@gmail.com> wrote:
> On Jul 20, 8:01=A0am, George Herold <gher...@teachspin.com> wrote:
>
>
>
>
>
> > Opamp current noise.
>
> > I got a call the other day from someone seeing =91too much=92 warm up
> > drift in the Johnson noise voltage from our Noise fundamentals
> > instrument.http://www.teachspin.com/instruments/noise/index.shtml
>
> > I had them send it back to me. =A0(A few hundred dollar learning
> > experience)
> > After much head banging, it turns out that the voltage noise of the
> > first stage opamp increases by ~3-5% in an exponential way with a ~10
> > =96 20 minute time constant. =A0(I haven=92t had time for a carful
> > measurement.)
>
> > Anyway, along the way I became worried about the current noise of the
> > opamp (opa134).
> > I=92m wondering if any one has an idea on how to measure this.
> > I think Phil H. mumbled something about a correlation technique when I
> > mentioned this a year or so ago. =A0But now I can=92t see how to make t=
hat
> > work.
>
> > My current idea (NPI) is to just switch in a second similar opamp.
> > Like this,
>
> > =A0 =A0switch
> > =A0 =A0 =A0|
> > =A0 =A0 =A0V =A0 |\
> > =A0 +-/.---|+\
> > =A0 | =A0 =A0 =A0| =A0>-+-->(output to nowhere)
> > =A0 | =A0 =A0+-|-/ =A0|
> > =A0 | =A0 =A0| |/ =A0 |
> > =A0 | =A0 =A0+------+ =A0 |\
> > =A0 +---------------|+\
> > =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0>-+-->(output to more gain,
> > =A0 R =A0 =A0 =A0 =A0 =A0 =A0 +-|-/ =A0| =A0 =A0filter, multiplier)
> > =A0 R =A0 =A0 =A0 =A0 =A0 =A0 | |/ =A0 |
> > =A0 R =A0 =A0 =A0 =A0 =A0 =A0 +------+
> > =A0 |
> > =A0GND
>
> > And measure the increase in noise for different R=92s.
> > Will this work?
> > Any other ideas?
>
> > Thanks,
>
> > George H.
>
> I'm trying to remember what my 24 bit sound card's noise density is,
> but 17nV/rtHz comes to mind. =A0If put a LT1115 preamp in there, you
> could get down below 2nV/rtHz and use that as an instrument to measure
> the noise density function.
>
> Plus you get a free tracking generator output along with it.- Hide quoted=
text -
>
> - Show quoted text -

Thanks Robert.  The issue isn't measureing the total noise, but
getting a measure of the fraction that is due to the opamp current
noise.  Measuring the opamp voltage noise is a snap... just short the
input.

George H.
```
```On Friday, July 20, 2012 9:51:17 AM UTC-7, George Herold wrote:

> ... Measuring the opamp voltage noise is a snap... just short the
> input.

Well, if the gain of the amplifier and its DC errors makes something
saturate, that can fail.  There's also opamp current noise at the
inputs, and that responds to some kinds of thermal effects,
which could be  your major issue.   Current noise doesn't
diminish when you short an input.

Shot noise in a transistor base current (bipolar transistor) goes
up with temperature, because current gain (beta) drops.
```
```On Jul 20, 1:49=A0pm, whit3rd <whit...@gmail.com> wrote:
> On Friday, July 20, 2012 9:51:17 AM UTC-7, George Herold wrote:
> > ... Measuring the opamp voltage noise is a snap... just short the
> > input.
>
> Well, if the gain of the amplifier and its DC errors makes something
> saturate, that can fail.

Well OK, but it works fine in this case.

There's also opamp current noise at the
> inputs, and that responds to some kinds of thermal effects,
> which could be =A0your major issue. =A0 Current noise doesn't
> diminish when you short an input.

Sure but the current noise contributes I^2*R^2 to the V^2 noise
output.

> Shot noise in a transistor base current (bipolar transistor) goes
> up with temperature, because current gain (beta) drops.

There are Jfets on the input of opa134.  They have a current noise
that rises with frequency. (and temperature)  I read somewhere what
this is due to, but have now forgotten it.  (too much beer.)

George H.

```
```George Herold wrote:
>
> Opamp current noise.
>
> I got a call the other day from someone seeing &#2013266065;too much&#2013266066; warm up
> drift in the Johnson noise voltage from our Noise fundamentals
> instrument.
> http://www.teachspin.com/instruments/noise/index.shtml
>
> I had them send it back to me.  (A few hundred dollar learning
> experience)
> After much head banging, it turns out that the voltage noise of the
> first stage opamp increases by ~3-5% in an exponential way with a ~10
> &#2013266070; 20 minute time constant.  (I haven&#2013266066;t had time for a carful
> measurement.)
>
> Anyway, along the way I became worried about the current noise of the
> opamp (opa134).
> I&#2013266066;m wondering if any one has an idea on how to measure this.
> I think Phil H. mumbled something about a correlation technique when I
> mentioned this a year or so ago.  But now I can&#2013266066;t see how to make that
> work.
>
> My current idea (NPI) is to just switch in a second similar opamp.
> Like this,
>
>    switch
>      |
>      V   |\
>   +-/.---|+\
>   |      |  >-+-->(output to nowhere)
>   |    +-|-/  |
>   |    | |/   |
>   |    +------+   |\
>   +---------------|+\
>   |               |  >-+-->(output to more gain,
>   R             +-|-/  |    filter, multiplier)
>   R             | |/   |
>   R             +------+
>   |
>  GND
>
> And measure the increase in noise for different R&#2013266066;s.
> Will this work?
> Any other ideas?
>
> Thanks,
>
> George H.

If you make the second amp a FET input one, it won't contribute current
noise.  Then the correlated part of the noise of the two amps will be
just the current noise times R.

So bung the two of them into a multiplier, and lowpass filter the
daylights out of the output.

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net
```
```On Fri, 20 Jul 2012 12:25:06 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Jul 20, 1:49&#2013266080;pm, whit3rd <whit...@gmail.com> wrote:
>> On Friday, July 20, 2012 9:51:17 AM UTC-7, George Herold wrote:
>> > ... Measuring the opamp voltage noise is a snap... just short the
>> > input.
>>
>> Well, if the gain of the amplifier and its DC errors makes something
>> saturate, that can fail.
>
>Well OK, but it works fine in this case.
>
>There's also opamp current noise at the
>> inputs, and that responds to some kinds of thermal effects,
>> which could be &#2013266080;your major issue. &#2013266080; Current noise doesn't
>> diminish when you short an input.
>
>Sure but the current noise contributes I^2*R^2 to the V^2 noise
>output.
>
>
>> Shot noise in a transistor base current (bipolar transistor) goes
>> up with temperature, because current gain (beta) drops.
>
>There are Jfets on the input of opa134.  They have a current noise
>that rises with frequency. (and temperature)  I read somewhere what
>this is due to, but have now forgotten it.  (too much beer.)
>
>George H.
>
>

Before 5 O'clock ?:-)

...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Jul 20, 9:51=A0am, George Herold <gher...@teachspin.com> wrote:
> On Jul 20, 12:30=A0pm, Robert Macy <robert.a.m...@gmail.com> wrote:
>
>
>
>
>
> > On Jul 20, 8:01=A0am, George Herold <gher...@teachspin.com> wrote:
>
> > > Opamp current noise.
>
> > > I got a call the other day from someone seeing =91too much=92 warm up
> > > drift in the Johnson noise voltage from our Noise fundamentals
> > > instrument.http://www.teachspin.com/instruments/noise/index.shtml
>
> > > I had them send it back to me. =A0(A few hundred dollar learning
> > > experience)
> > > After much head banging, it turns out that the voltage noise of the
> > > first stage opamp increases by ~3-5% in an exponential way with a ~10
> > > =96 20 minute time constant. =A0(I haven=92t had time for a carful
> > > measurement.)
>
> > > Anyway, along the way I became worried about the current noise of the
> > > opamp (opa134).
> > > I=92m wondering if any one has an idea on how to measure this.
> > > I think Phil H. mumbled something about a correlation technique when =
I
> > > mentioned this a year or so ago. =A0But now I can=92t see how to make=
that
> > > work.
>
> > > My current idea (NPI) is to just switch in a second similar opamp.
> > > Like this,
>
> > > =A0 =A0switch
> > > =A0 =A0 =A0|
> > > =A0 =A0 =A0V =A0 |\
> > > =A0 +-/.---|+\
> > > =A0 | =A0 =A0 =A0| =A0>-+-->(output to nowhere)
> > > =A0 | =A0 =A0+-|-/ =A0|
> > > =A0 | =A0 =A0| |/ =A0 |
> > > =A0 | =A0 =A0+------+ =A0 |\
> > > =A0 +---------------|+\
> > > =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0>-+-->(output to more gain,
> > > =A0 R =A0 =A0 =A0 =A0 =A0 =A0 +-|-/ =A0| =A0 =A0filter, multiplier)
> > > =A0 R =A0 =A0 =A0 =A0 =A0 =A0 | |/ =A0 |
> > > =A0 R =A0 =A0 =A0 =A0 =A0 =A0 +------+
> > > =A0 |
> > > =A0GND
>
> > > And measure the increase in noise for different R=92s.
> > > Will this work?
> > > Any other ideas?
>
> > > Thanks,
>
> > > George H.
>
> > I'm trying to remember what my 24 bit sound card's noise density is,
> > but 17nV/rtHz comes to mind. =A0If put a LT1115 preamp in there, you
> > could get down below 2nV/rtHz and use that as an instrument to measure
> > the noise density function.
>
> > Plus you get a free tracking generator output along with it.- Hide quot=
ed text -
>
> > - Show quoted text -
>
> Thanks Robert. =A0The issue isn't measureing the total noise, but
> getting a measure of the fraction that is due to the opamp current
> noise. =A0Measuring the opamp voltage noise is a snap... just short the
> input.
>
> George H.

I didn't mean total noise, but true "noise density function". The
soundcard easily finds you noise density. I use it all the time.

How do you do it now, since at the output you have both current and
voltage noise.

Floundering around a bit here, but I perceive simultaneous measurement
[soundcard has two channels at 192kS/s] measure at input terminal
measure at output terminal. then compare. Voltage noise is new at the
output, but current noise also appears at the input, so with
correlation you can discern which is which.

```